Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers so as to define a p-n junction. When a bias is applied across the p-n junction, holes and electrons are injected into the active layer where they recombine to generate light in a process called injection electroluminescence. Light may be emitted from the active layer through all surfaces of the LED.
As most LEDs are nearly monochromatic light sources that appear to emit light having a single color, light emitting devices or lamps including multiple LEDs that can emit light of different colors have been employed to produce white light. In these devices, the different colors of light emitted by the individual LEDs combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white.
As an alternative to combining individual LEDs to produce light emitting devices having a particular light emission spectrum, luminescent materials, or phosphors, may be used to control the color of light emitted from LEDs. A phosphor may absorb a portion of the light emitted from an LED at a given wavelength and re-emit the light at different wavelength via the principle of photoluminescence. The conversion of light having a shorter wavelength (or higher frequency) to light having a longer wavelength (or lower frequency) may be referred to as down conversion. For example, a down-converting phosphor may be combined with a blue LED to convert some of the blue wavelengths to yellow wavelengths in order to generate white light.
A widely used phosphor for white light generation is yttrium aluminum garnet (YAG), which may be doped with cerium (Ce), e.g., Y3-xCexAl5O12 or YAG:Ce. This yellow phosphor may be used in combination with a blue LED to produce white light. Compared to other phosphors based on silicates and sulfides, for example, YAG:Ce has a relatively high absorption efficiency of blue excitation radiation, a high quantum efficiency (greater than 90%), good stability in high temperature and/or high humidity environments, and a broad emission spectrum.
Blue LEDs, which typically emit light over a wavelength range of about 425 nm to 475 nm, may exhibit an increased radiant flux at shorter emission wavelengths (e.g., less than 460 nm). Also, the decrease in radiant flux observed at increasing temperatures may be less severe at shorter wavelengths, and the current dependence of the radiant flux tends to improve at shorter wavelengths. For these reasons it would be advantageous to employ blue LEDs that emit at shorter wavelengths in solid state light emitting devices. However, the most common yellow phosphor used in phosphor-converted LEDs, YAG:Ce, is less efficiently excited as the wavelength decreases. FIG. 1 shows a photoluminescent excitation spectrum of an exemplary YAG:Ce phosphor as measured by a fluorescence spectrophotometer. The plot shows the normalized (relative) intensity of the phosphor emission as the wavelength of the incident light is varied. The relative intensity drops from about 95% to about 80% as the wavelength decreases from about 457 nm to about 440 nm.